Why does a comet grow a coma and tails only when it comes near the Sun?

Comets are icy objects made of dust and ice that travel round the Sun. They are leftovers from the birth of the solar system, about 4.6 billion years ago.

Most comets spend their lives far out in the cold. Short-period comets, which take less than 200 years to go round the Sun, come from the Kuiper Belt, a region of icy bodies at the edge of the solar system. Long-period comets come from the even more distant Oort Cloud, and some take more than 250,000 years to complete one orbit. Now and then the gravity of a planet or a passing star nudges a comet onto a path that brings it towards the Sun.

The solid, frozen centre of a comet is called the nucleus, and it is usually less than about 16 kilometres across. Far from the Sun, that is all there is to see. But as a comet comes closer, the heat turns some of its ice into gas, and a huge, fuzzy cloud called the coma forms around the nucleus. The comet then grows two tails: a white, gently curving tail of dust, and a bluish tail of electrically charged gas. The tails can stretch for millions of kilometres, and they grow longer the nearer the comet comes to the Sun.

Some comets return on a regular timetable. Halley's Comet, the most famous, comes back about every 76 years. Spacecraft have even visited comets: NASA's Stardust mission collected dust from one, and the Rosetta spacecraft put a lander on another.